[SalesForce] Restrict contact lookup to show only contacts related to account in case object

Contacts that do not have an Account associated with them are Private (to the person who created it) so you won't be able to 'see' them in the Lookup.

Some more info here:

https://na13.salesforce.com/help/doc/en/contacts_sharing.htm

http://success.salesforce.com/questionDetail?qId=a1X30000000JDN7EAO

If you are entering the Account Name in the Contact Name lookup it won't return any results, unless there is a coincidental match of your search string on a Contact Name.

I agree with @eyescream that it is just how Salesforce works. There's an Idea: Lookup Filter Should Automatically Display Values on click of lookup button.

When it is a required filter your search results will be limited to those that match search string and the filter. If it is an optional filter it will initially restrict the results to those that match the search string and the filter, but will give you an opportunity to show all that match the search string, disregarding the filter.

You can add help text to the lookup window, from the filter configuration page, to explain to the user that they still need to search, so there is less of a chance for confusion. I could definitely see a scenario where the user clicks the lookup and thinks that the recently viewed list is the actual total list, if they are less familiar with Salesforce.